Various methods have been studied to carry out a solid-liquid separation for liquid to be treated using membrane modules provided with a separation membrane, such as a microfiltration membrane and an ultrafiltration membrane, in water treatment of sewage, drainage, etc. It is possible to obtain treated water of high quality when a filtration process of liquid to be treated is carried out using a separation membrane.
When a solid-liquid separation of liquid to be treated is carried out using a filtration membrane, since clogging of separation membrane due to suspended substances (SS) progresses as the filtration process continues, problems, such as reduction of filtration rate and increase in pressure difference between membranes, occur. In order to recover from such state, a technique is known in which a diffuser is disposed below a membrane separation device and air is diffused during a filtration process or when a filtration process is stopped.
A method for carrying out a stable solid-liquid separation using a hollow fiber membrane module in which a hollow fiber membrane of a sheet shape is fixed to a rectangular housing using a resin so as to maintain a constant distance between adjacent membrane modules and air is bubbled from a diffuser so that flow of liquid to be treated is uniformly directed to the membrane modules, is disclosed in Japanese Laid-Open Patent Application, No. Hei 5-261253, Japanese Laid-Open Patent Application, No. Hei 6-342, Japanese Laid-Open Patent Application, No. Hei 6-340, etc.
A method is known in which a plurality of membrane modules are fluid-tightly connected to a tubular catchment header having holes for connecting to the membranes modules so that a constant distance is maintained between adjacent membrane modules and filtered liquid is collected and taken out from the membrane modules.
Various catchment headers have been proposed, such as one in which a resin flat plate having holes for connecting to modules is welded to resin pipes, one in which a metal thick plate having holes for connecting to modules is welded to a metal thin plate which is bent to be in a U-shape, and one in which a metal thick plate having holes for connecting to modules is combined with a resin water passage part.
Although one in which a resin flat plate having holes for connecting to modules is welded to resin pipes can be easily processed as compared with a metallic one, the mechanical strength of a resin pipe which forms a catchment part is weaker than that of a metallic pipe of the same shape.
For this reason, when a catchment header is formed using a resin pipe, the outer diameter of a catchment header has to be made large in order to resist bending stress applied to the catchment header during a bubble washing, etc., for a long period of time. Accordingly, a portion of the header which is not directly involved with a filtration process becomes large, and hence it is difficult to improve the catchment efficiency of a membrane.
Also, when sludge attaches to the surface of a membrane due to some kind of problem and a membrane separation device is pulled out of water in order to solve the problem, a bending stress which is larger than normal may be applied to the catchment header and the header may be broken. Accordingly, it is necessary to increase the diameter of the catchment header so as to withstand a stress which is larger than normal to avoid the breakage, and this leads to further decrease in the catchment efficiency.
Moreover, although there are a catchment header in which a metal thick plate having holes for connecting to modules is welded to a metal thin plate which is bent to be in a U-shape, and in which a metal thick plate having holes for connecting to modules is combined with a resin water passage part, it is necessary to carry out a process for continuously forming a plurality of holes for connecting to modules in a metal thick plate for these catchment headers and such a process is quite difficult to perform. Furthermore, as for welding to a metallic thin plate, it is not easy to perform continuous welding without causing distortion in a longitudinal direction and, for example, 5 to 6 mm flexure would be generated in both horizontal and vertical directions due to thermal distortion of welding when a 70 cm catchment header is produced by welding. Accordingly, it is difficult to orderly arrange a plurality of membrane modules with a distorted catchment header.
In addition, there is a danger that welding will become insufficient and the welded portions may be separated during use of a long period of time, causing leakage of liquid.
The present invention has been achieved in consideration of solving the above-mentioned problems, and objects thereof include to provide a light-weight and compact catchment header which is excellent in workability and is capable of carrying out a solid-liquid separation process using a separation membrane in a stable manner for a long period of time.